This invention relates to imaging systems, and more particularly, to improved electrostatographic developing materials, their manufacture and use.
The formation and development of images on the surface of photoconductor materials by electrostatic means is well known. The basic xerographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Several methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely divided toner particles electrostatically coated thereon is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the carrier particles is so selected as to triboelectrically charge the toner particles to the desired polarity. As the mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier and excess toner are then recycled. This technique is extremely good for the development of line copy images.
Another method of developing electrostatic images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles are carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carrier into a brush-like configuration. This "magnetic brush" is engaged with the electrostatic image-bearing surface and the toner particles are drawn from the brush to the latent image by electrostatic attraction.
Still another technique for developing electrostatic latent images is the "powder cloud" process as disclosed, for example, by C. F. Carlson in U.S. Pat. No. 2,221,776. In this method a developer material comprising electrically charged toner particles in a gaseous fluid is passed adjacent the surface bearing the electrostatic latent image. The toner particles are drawn by electrostatic attraction from the gas to the latent image. This process is particularly useful in continuous tone development.
Other development methods such as "touchdown" development, as disclosed by R. W. Gondlach in U.S. Pat. No. 3,166,432, may be used where suitable.
The developed image can then be read or permanently affixed to the imaging surface of the photoconductive substrate if this imaging surface is not to be reused. In the event that the imaging surface is of a reusable material and is to be used in preparation of subsequent electrostatographic copies, the developed image can be transferred to another substrate, such as paper, and then permanently affixed thereto. Various techniques have been devised to permanently affix this toner image to its substrate including overcoating the toner image with a transparent film, and solvent or thermal fusion of the tone particles to the substrate material. The energy requirements involved in thermal fixation of the toner are considerable since these thermoplastic toner materials often require temperatures in the range of 350.degree.-400.degree. F and higher to fuse them to the substrate. Thus, a substantial reduction in the fusion temperatures of the toner would result in a corresponding reduction in energy requirements of such an imaging process. Any reduction in the fusion temperature of the toner would also permit lowering the operating temperatures within the copier and, therefore, reduce the demands placed upon the temperature control unit within such an apparatus.
Although some of the foregoing development techniques are employed commercially today, the most widely used commercial electrostatographic development technique is the technique known as "cascade" development. A general purpose office copying machine incorporating this development process is described in U.S. Pat. No. 3,099,943. The cascade technique is generally carried out in a commercial apparatus by cascading a developer mixture over the upper surface of an electrostatic latent image-bearing drum having a horizontal axis. The developer is transported from a trough or sump to the upper portion of the drum by means of an endless belt conveyor. The developer is cascaded downward along a portion of the surface of the drum into the sump and is subsequently recycled through the developing system to develop additional electrostatic latent images. Small quantities of toner are periodically added to the developing mixture to compensate for the toner depleted by development. This process is then repeated for each copy produced by the machine and is ordinarily repeated many thousands of times during the usable life of the developer.
Thus, it is apparent from the description presented above, as well as in other development techniques, that the toner is subjected to mechanical attrition which tends to break down the particles into undesirable dust fines. Toner fines are detrimental to machine operation because they are extremely difficult to remove from reusable imaging surfaces and also because they tend to drift to other parts of the machine and deposit on critical machine parts such as optical lenses. The formation of fines is retarded when the toner contains a tough, high molecular weight resin which is capable of withstanding the shear and impact forces imparted to the toner in the machine. Unfortunately, many high molecular weight materials cannot be employed in high speed automatic machines because they cannot be rapidly fused during a powder image heat fixing step. Attempts to rapidly fuse a high melting point toner by means of oversized high capacity heating units have been confronted with the problems of preventing the charring of paper receiving sheets and of adequately dissipating the heat evolved from the fusing unit or units. Thus, in order to avoid charring or combustion, additional equipment such as complex and expensive cooling units are necessary to properly dispose of the large quantity of heat generated by the fuser. Incomplete removal of the heat evolved will result in operator discomfort and damage to heat-sensitive machine components. Further, the increased space occupied by and the high operating cost of the heating and cooling units, often outweigh the advantages achieved by the increased machine speed. On the other hand; low molecular weight resins which are easily heat fused at relatively low temperatures are often undesirable because these materials tend to form thick films on reusable photoconductor surfaces. These films tend to cause image degradation and contribute to machine maintenance down time. In addition, low molecular weight resins tend to form tacky images on the copy sheet which often offset to other adjacent sheets. Further, toner particles containing low molecular weight resins tend to bridge, cake, and block in the shipping container as well as in the xerographic machine. Also, the toner material must be capable of accepting a charge of the correct polarity when brought into rubbing contact with the surface of carrier materials in cascade, magnetic brush or touch-down development systems. Some resinous materials which possess many properties which would be desirable in xerographic toners dispense poorly and cannot be used in automatic copying and duplicating machines. Other resins dispense well but form images which are characterized by low density, poor resolution, or high background. Further, some resins are unsuitable for processes where electrostatic transfer is employed. Many thermoplastic materials, such as those presently in use in electrostatographic toners, have traditionally been difficult to mold or form because of unfavorable rheological properties. probably the most widely accepted technique for modification of these thermoplastics to facilitate forming of such materials has been the inclusion of certain additives in said materials designed to reduce their melt viscosity. These additives, generally referred to in the art as "plasticizers", include non-volatile organic liquids or low melting solids, e.g. phthalate, adipate and sebacate esters and aryl phosphate esters.
The interaction of the plasticizer and the resin in the melt results in a marked improvement in the composition's rheological properties by affecting a shift in both the glass transition temperature and fusion points of the composition to lower temperatures. This shift in the glass transition temperature of the thermoplastic materials used as electrostatographic toners, is substantial, can cause these discrete, finely-divided toner particles to form larger agglomerates. This agglomeration, more commonly referred to in the electrostatographic art as "blocking", adversely affects the free flow characteristics of the toner. For example, in cascade-type development systems, the momentum of these larger toner particles as they tumble over the imaging member can exceed the attractive forces of the latent image and, therefore, result in failure of development of the latent image by these larger toner particles. Since most thermoplastic materials are deficient in one or more of the above areas, there is a continuing need for improved toners and developers.